Spring-controlled mechanism



Feb. 13, 1940. G. CARWARDINE 2,190,400

SPRING-CONTROLLED MECHANISM Filed Oct. 14, 1937 8 Sheets-Sheet l Feb. 13, 1940. e. CARWARDINE 2,190,400

SPRING-CONTROLLED MECHANISM Filed Oct. 14, 1957 8 Sheets-Sheet 2 477' P/VEXS Feb. 13, 1940. e. CARWARDINE SPRING-CONTROLLED MECHANISM 8 Sheets-Sheet 3 Filed Oct. 14, 1937 Feb. 13, 1940. gA w m 2,190,400 SPRING-CONTROLLED,MECHANISM Filed Oct. 14, 1937 .8 Sheets-Sheet 4 Fig. 5.

Arm/YEYS Feb. 13, 1940. G. CARWARDINE I 0 2,190,400

SPRING-CONTROLLED MECHANISM Fil'ed 0ct 14, 1937 8 Sheets-Sheet 5 75/A VE/V OR AFC/14mm 00v BY ATTORNEYS Feb. 13,1940. G. CARWARDINE 2,190,400

SPRING-CONTROLLED MECHANISM 8 Sheets-Sheet 6 Filed Oct. 14. 1957 INVE/VTDI? VI/IIIIIIIIIIIIA 14770 NEW 8 Sheets-Sheet 7 Filed 001;. 14, 1937 TOP/Vtfg mil-M Feb. 13, 1940;

G. CARWARDJNE SPRING-CONTROLLED MECHANISM Filed Oct. 14, 193'? 8 Sheets-Sheet 8 Patented Feb. 13, I

PATENT OFFICE spams-commune MECHANISM George Carwardine, Bath, England, assignorof one-third to Herbert Terry & Sons Limited, Redditch, England, a company'of Great Brit- Application October 14, 1937, Serial No. 169,014

In' Great Britain 'ctober 31, 1936 In servo-assisted mechanism, such as servo-assisted brake mechanism of a -motor vehicle, in which two surfaces have to be moved into and a out of engagement with each other and have to "6 be forced tightly together when they are in engagement,' it is often. important that the servo device be brought intooaction in strict timed relationship with the relative movement of the surfaces. Thus, in the case of servo-assisted ve- '10hic1e brakes it is usually desirable for the servo device to be rendered operative almost exactly at the moment when the braking surfaces make contact and for the servo device thereafter to exert a rising force effort increasing the pressure between the surfaces in step with the operation by the driver of the brake lever or pedal. Now it will be appreciatedthatinthe event of the surfaces becoming worn, thepoint in the movement of the lever or pedal at which the braking surfaces make contact will change, so that unless the servo device is to get out of phasein its operation relatively to the time taken to bring the surfaces into contact, the servo device must be arranged to T start delivering its contribution to the force necessary for increasing the pressure between the surfaces later than in the case where the surfaces are unworn. An aim of the invention is,

therefore, to provide servo mechanism for operation in association with friction surfaces which has no effect on the operations of bringing the surfaces into and out of engagement, but which, as soon as the surfaces make contact, and only when they make contact, expends energyv in pressing the surfaces tightly together. a

According to the invention, mechanism, provided with a servo device for exerting pressure between two engaging surfaces and with a forcetransmitting member that takes part in bringing the surfaces into and out of engagement, is fur- 14o nished with a gripping mechanism which is so associated with the force-transmitting member and the servo device, that the clutch permits the servo device to remain inoperative as the forceexperiences the reaction due to the engaging .sur-

faces, and provide a connection through which or against the reaction of which the servo device acts to increase the pressure between the surfaces. When the mechanism is incorporated in the braking system of a vehicle, the brake pedal or lever is arranged to undergo an initial movement in applying the brakes during which the force-transmitting member'is moved through the medium of the pedal lever to bring the braking surfaces into engagement. Then, when the force- H "transmitting member experiences the resistance due to contact between the surfaces, further movement of the pedal lever relatively to the force-transmitting member results in the operation of the g ipping mechanism. In this case the said transmitting 'member is connected through the medium of the usual brake linkage to the brake shoes, and inthe initial movement there is no resistance to the movement of the force-transmitting member except that due to friction and that due to the light springs in the brakes which tend always to withdraw the brake shoes from the drums.

In order that the invention may be clearly understood and readily carried into effect, various mechanisms, in accordance therewith, for controlling vehicle brakes will now be described by Way of example with reference to the accompanying drawings, in which Figure 1 is a side elevation of one mechanism partly in section; 1

Figure 2is a side elevation of a second mechanism;

Figure 3 is a sectional plan of part of a modification of the mechanismqof Figure2;

, Figure 4 is a side elevation of a third mechar nism Figure 7 shows a detail of the mechanism of Figure 6 to an'enlarged scale;

Figure 8 shows the view seen when looking in the direction of the arrow VIII in Figure 7;

Figure 9 is a sectional elevation of a fifth mechanism;

Figure 10 is a sectional elevation of a sixth mechanism;

Figure llis a side elevation of 'a modification of the mechanism of Figure 10;

Figure 12 is a diagrammatic elevation of part of a further modification of the mechanism of Figure 2.

Figures, 13, 14 and 15 are diagrammatic perspective views each showing the mechanism of Figure 1 connected to brakes through a linkage,

these figures respectively showing the parts in three stages in their operation.

In the mechanism of Figure 1, the arrangee ment is such that the vehicle brakes are applied through the medium of a linkage including a link b pivoted to a force-transmitting member in the form of a lever 2 which in turn is under the control of an operating element constituted by a pedal lever I. When the latter is moved so as to apply the brakes it is swung about the axis of a spindle B through a small, initial angle as the brake shoes are brought into contact with the brake drums. During this initial movement, the lever 2 moves as if integral with the lever I, the lever 2 being held against a stop 3 on the lever i by a spring i. As soon, however, as the brake surfaces engage, there is substantial resistance to further movement of the lever 2. The result is that the continued movement of the pedal lever I causes the spring i'to extend while relative movement occurs between the levers I and 2 resulting in the engagement of a ratchet clutch which puts a servo device into connection with the brake operating linkage. This ratchet clutch comprises a lever l carrying teeth 8 and a member Q carrying an arcuate row of teeth It. The

teeth 8 are drawn into engagement with the teeth It by a link II which, as thepedal lever I swings away from the lever 2, draws the lever I about its pivot '10., at theupper end of the lever 21, towards the member 9. The clutch member ii is pivoted on the spindle I5 and when engaged by the clutch lever '1 is swung as a unitary assembly with the pedallever I and force-transmitting lever 2 about the common axis of the spindle 6.

The servo device comprises a telescopic strut i2, the two parts of which are formed respectively with plane surfaces bearing respectively against a fixed pivot pin, l3 and againsta pivot pin M mounted on the clutch member 9. A compression spring I5 tends always to expand the strut. Before the ratchet clutch is engaged, the servo mechanism is in the position shown in Figure l with the axis of the pin it beneath the plane containing the axes of the spindle Ii and pin it and the spring I 5 tending to swing the telescopic strut anti-clockwise about the axis of the pin it (by reaction on the clutch member 9) Such anti-clockwise motion is prevented by engagement with a plate It forming part of the fixed supporting structure for the mechanism. It will be seen, therefore, that during a very short period at the beginning of the-anti-clockwise motion of the clutch member 9, the spring I5 is compressed as the pin I4 is raised to the deadcentre position in alinement with the pin I3 and the spindle 6. Then, when the pedal lever I has been moved far enough to cause the clutch memoer ii to carry the strut beyond the dead-centre position, the spring I5 is able to expand. Further movement of the brake pedal I is therefore accompanied by a corresponding amount of expansion of the servo spring I5 which, as it expands, plays the predominant part in increasing the pressure between the braking surfaces. The motion of the pedal lever I and the expansion of the spring I5 necessary to bring about this increase in pressure between the braking surfaces is rendered possible by the straining of the linkage connected between the lever 2 and the brake shoes.

After the pin M is passed over the dead-centre position to a position such as that indicated by A in Figure 1, the force due to the driver provides only a verysmall proportion of the totalforce serving to press the braking surfaces tightly together. As soon, however, as the driver relaxes his pressure on the pedal lever I the energy stored in the strained linkage'is able to overcome the servo spring lfiand return the parts to their initial positions.

- This action will be better understood with" reference to Figures 13 to 15 which. show diagrammatically one method of connecting the mechanism of Figure l to the two rear brakes of a vehicle. Thesefigures show the link 5 connected to one of two levers 2m fixed to a shaft: v

The other 268 mounted to turn about itsaxis. lever Elli? is connected to the middle one of three levers 286 fixed to a shaft 2135 mounted to turn about its axis. The two outer levers 205 are connected respectively by links 284 to levers 203 by which the pairs of brake shoes 208 are operated. The links 5, 2i)? and 2M, together with the levers. 2m, 20s and 203, and the shafts 2min. and 285, constitute the aforesaid brake operating linkage. When the parts are in the positionslof Figure 13, the servo mechanism is, in the state shown in Figure land the brake shoes 20!); are

separated from their associated brake drums 28 I, the shoes being held in these, positions'by light. springs 2&2, eachacting between its respective pair of shoes. In the first part of the movement of the brake pedal I to'apply the brakes, the pedal is moved from the position of Figure 13 to that of Figure 14, so that the shoes 2% are brought into I contact with the inner surfaces of the brake drums Zil I In this movement the only resistance is that due to the light springs 2G2 and that due to friction. This resistance is not sufficient to cause the spring 4 to stretch appreciably, so that the pedal lever I and the lever 2 move as a unit about the axis 8.

denly increases veryconsiderably so that,'upon a further movement of the pedal lever I, the lever-2 is momentarily held from following while shoes 200 being prevented from further movement by their engagement with the brake drums 2m, is permitted only ,by the resilience in the brake linkage. movement of the member 9, the telescopic'strut -When, however, the brakev shoes 280 make contact with the brake drums 21H,

the resistance to movement of the lever 2 sud- The clutch member 58 now also partakes In the first small fraction of the T Then, due to this engagement, the

I2 is swung over its dead centre position. Then] the relaxing servo spring I5 exerts, due to1the increasing obliquity of the telescopic strut I2,'

a rising force effort'assisting the operator injincreasing the pressure between the contacting brake surfaces. positions such as shown in Figure 15.

The parts eventually reach The linkage shown in Figures 13 to 15 represents the kind of linkage usually employed in mechanical brake systems. Being metallic, it It obeys Hookeslaw has considerable, resilience. and may be regarded as a simple spring. There-r fore, as the servo device is moved to theposition of Figure 15, the spring constituted by the linkage is strained, one end'of this linkage'being held firm by the engagement of the drums 2! with the shoes 29%. of torsion in the shafts 2G5 and 288, stretch in the links 264, 201 and 5 andbending in the This straining is made up.

levers 203, 206 and 2H1." Such straining occurs normally in the operation of mechanicalbrake systems, in the absence of servo assistance. The latter is provided to relieve the driver of much of the effort that would otherwise be necessary to effect the straining of the linkage. I

At any point in the movement of the servo spring l5 on the effective sideof its dea'dcentre position, .it exerts a force which is somewhat less than the opposing force exerted by the strained linkage at that point. The difference is made up by the effort exerted by the driver on the pedal lever .I. moved, the strained linkage, being stronger than the servo device, returns the servo spring to the dead centre position. In the condition of "Figure 15, for example, the strained linkage may be regarded as pushing hard against the brake shoes at one end to cause considerable friction between the brake surfaces and as pushing hard against the combined effort of the servo spring and the driver at the other end. When the driver removes his effort, the equilibrium is upset and the strained linkage relaxes while it returns the servo spring 15 to dead centre. the force due to the relaxing linkage decreases but'the'opposingforce due to. the servo spring also decreases and remains less than the force due to the strained linkage until the dead centre position is reached.

In some vehicles the brake linkage may not be such as to permit sufficient movement of the lever 2 and a correspondingtransference of energy from the servo spring 15 and from the drivers foot to the brake linkage. Then, a further 'spring,such as the spring 209 indicated by dotted lines in Figure 15 can be inserted to formpart of the brake'linkage. The insertion of this spring is tantamount to putting one spring in series with another, the brake linkage being, as mentioned above, in the nature of a spring.

It maybe noted that, as soon as the pin I 4 (Figure 1) passes beyond the dead-centreposition duringthe application of the brakes, the force dueto the pressure of the drivers foot is transmitted to the link-5 through the link I l and through the assembly'consisting of the parts 2,

I and 9. The rows of teeth 8 and I areso shaped that duringthe initial movement to bring the pin M to the dead-centre position, they remain in engagement and that after the deadcentre position has been passed the expanding spring 15 keeps them in engagement. When the strained linkage is allowed to return the strut I to the dead-centre position the two parts of the ratchet clutch are still :held together, as in the brake-applying movement, on account of'the opposition of the stresses respectively in the linkage and in the spring l5 tending to turn the members 2 and 9 in opposite directions, the teeth be ing so shaped that the spring 4 cannot separate them. When the dead-centre position is passed on the return movement the forces, by then very small, due respectively to the relaxing linkage and the spring l5 are in the same direction so that the spring 4 is able to separate the two. parts of the clutch.

As wearin the braking surfaces takes place, the point in the brake-applying movement of the levers I and 2 at which the ratchet clutch is brought into action varies, the teeth 8 engaging the row of teeth 9 further and further along that row.

Referring now to the mechanism of Figure 2,

when an operating element .or .pedal lever ll in Thus, when the drivers efiort is re 25 which slide freely on a square tube 25.

It is true that 7 that mechanism is caused to undergo its small initial movement prior to the engagement of the braking surfaces, at force-transmitting member 'or lever i8 is swung through the medium of links H! which connect the lever [B with a unit 20, 5 2 I ,which is, in turn, pivoted to the lever ll. The lever 18 is connected to the brake shoes through the medium of a linkage including a compression link 22., When the mechanism replaces the servo mechanism shown in Figures 13 to 15, the link 22 is arranged to replace the link 5 and to push on the corresponding lever 2H3. The unit 29, 2| comprises a pair of links 20 and a pair of levers 2|, the latter being held, in the initial movement, by a spring 23 against stops 24 on the links 29. The levers 2! carry friction shoes When the point at which the braking surfaces come into contact is reached, the continued movement of the pedal lever ll results in relative movement between the links 20 and the levers 2i,

Which hitherto have behaved as a unit, the springs 23 being extended while the lever l8 remains substantially stationary. The movement of the levers 2| about the axis at which they are con- '-25 nected to the links 28 results in the shoes 25 being moved. closer together and gripping the square tube 26. I

When the square tube 25 has been firmly gripped by the shoes 25, continued movement o'f"'30 the pedal lever ll causes the links 20 to thrust the shoes 25 and the tube 26 as a unit to the right. The tube 26 engages slidingly within a guide 25a and fits over the actuated rod 2! of a spring servo 'unit S, including a servo spring S which initially lies'slightly to one side of a dead-centre position with respect to axes A and A of members S and S between which the spring S is connected and which have arcuate surfaces S and S which roll on each other as the rod 21 is .40 moved. longitudinally. The members S and S are pivoted respectively to a fixed rod S and to the rod 21 which is constrained to move in alignment with the rod S During a small initial part of the movement to the right of the tube 26, the rod 21 is pulled to the right by springs 28 connected between pins 29 and 30 mounted respectively on the tube 26 and the rod 21. The

springs 28 are strong enough not to expand apbetweenthe tube 26 and the rod 21 but the servo 5155 spring forces the pin 36 hard against the end of the tube 26, transmitting force through the latter, the shoes 25, the links l9 and the lever l8 to the brake linkage. The servo spring then plays the predominant part in straining the linkr5 age so as to force the braking surfaces tightly together. When the drivers comparatively small contribution to the straining of the brake linkage has been removed, the force stored in the latter is sufilcient to overcome the servo springs: 5 and return the parts to their initial positions.

A small yielding catch M is provided to locate the tube 2% in the initial or fully off position of the servo unit. i

The springs 28 are provided in case circum-i-m stances should arise that cause the servo rod 21 to be shottothe right, when the pedal lever 41 is in the initial position shown in Figure 2 and the shoes 25 are consequently not gripping the tube 26. If this should happen the brakes could e7 not be applied in the absence of the springs 28 because the shoes would simply grip the rod 26 and prevent the pedal lever H from being moved further. The springs 28, however, allow the tube 26 to be moved to the right relatively to the rod 2'! if these abnormal conditions arise, the brakes being applied by foot pressure without any help from the servo unit S.

Figure 3 shows a modification of the construction in Figure 2 in which the shoes 25 are replaced by wedge members 32 hearing on each side of the square tube 2%. The wedge members 32 are connected to the pedal lever ill by a pair of links 33, the latter corresponding to the links 20. During the initial movement to bring the controlling surfaces into contact, thrust is transmitted, from the links 33 and the wedge members 32, through a spring 34 to an external member 35 which in turn is connected by a pair of links 36, equivalent to the links it, to the lever l8. During this movement, the Wedge members 32 and.

the external member 35 do not move relatively to each other, but as soon as the braking surfaces engage, the spring 33 yields so that the wedge members 32 move to the right relatively to the external member 35. This relative movementresults in the inclined surfaces on the wedge members 32 closing upon inclined surfaces on the external member 35 so that rollers 3? are gripped between the surfaces and the wedge members 32 are locked to the tube it.

In the mechanism of Figures 4 and 5 the moving parts are supported between two plates 38 which, in turn, are connected through the medium of bushes 39 to the vehicle chassis. A spindle 40 carrying a spacing sleeve extends between the plates 38, and the sleeve ll serves as a fulcrum for a rotatable force-transmitting member t2 connected by linkage, including a link G3, to the brake shoes. The link corresponds to the link 5 in Figures 13 to 15. The rotatable member 42 is formed, as shown in Figure 5, with a flange portion and a bush portion through which the sleeve M passes, while an annular plate 44 is screwed on to the end of the bush portion remote from the flange portion. A pedal lever 45 is secured to a flange member at, pivotally mounted upon the outside of the bush portion of the rotatable member The flange member 46 is connected by pins ll to a'flat ring 28, the arrangement being such that the flange member and ring must always rotate together about the axis of the sleeve ll but are capable of independent relative movement along that axis.

During the initial movement of the brake pedal to bring the braking surfaces into contact, the flange member 35 rotates, carrying the rotatable member 2 with it, these two parts being drawn together by springs (notv shown). However,

when the resistance due to the engagement. of j the braking surfaces holds up the member 52, these springs yield, while the relative movement between the flange member ti; and the member 42, resulting from further movement of the pedal lever 45, causes balls 38, located in a ball race 59, to travel up the inclined surfaces of conical members 5i and 52, carried respectively by the rotatable member 2 and the flange member -13. The latter is thus forced axially to the right, as viewed in Figure 5, relatively to the ring 38, and a clutch element 53, bearing friction material 5 3 on each face, is gripped firmly between the flange member t6 and the ring 68, the latter being prevented from moving axially to the right by thrust ball bearings, in a ball race 55, which, in turn,

bear against the annular plate 44. The clutch element 53 is, as shown in Figure 5, pivotally .mounted on a sleeve integral with the flange member 46.

As soon as the clutch element 53 has been firmly gripped, the continuedmovement of .the pedal lever 35 causes a pair of pins 56, carried between pairs of ears forming part of the clutch element to pass over dead-centre positions relatively to a pair of pins 51 carried respectively by a pair of double levers 58 pivoted at'their upper ends about fixed axes 59 on the plates 38. A pair of struts til extend, as shown in Figure 4, between the pins 56 and 51, and as soon as the dead-centre.

position has been passed, a spring BI is able to exert a turning moment on the clutch element 53, so that during continued movement of the pedal lever 65 the relaxing spring 6| assists ina substantial manner the forcing together of'the braking surfaces. The struts 60, through which the spring force is transmitted to thegclutch element are formed, as shown in Figure 4, with aper tures, the surfaces of-which have'flat portions against which the pins and 5? bear. As in the previously described examples, when the pedal lever 45 is released, the strained brake linkage is able to overcome the spring 6| and, in re laxing, returns the clutch element 53 to) and be- I v In the form of the invention shown in Figures 6 to 8, a pedal lever 62 is arranged to be swung in the direction of the arrow Bin 'order to apply the brakes. The pedal lever 52 is divided Within the easing into two parallel arms 62abetween which are pivoted a telescopic strut 63 and a connecting member it. The latter comprises two parallel parts 64a joined by transverse parts Mb carrying respectively a pair of pins 1.65 in recesses, themember 64 being in the form of the letter H when viewed in plan. The parts 64d of the connecting member 64 are formed with parallel slots 66 (see particularly Figure 7) which receive respectively pins 6'! that are carried, in

turn, respectively by two arms. 68a. forming, part of a force-transmitting lever 68 that is connected through an appropriate linkage (like the linkage Mid-4 H3 shown in Figures 13 to 15) to the brake A compression spring 69 tends always to shoes. expand the telescopic strut G3 which is also pivoted between the arms 68a and, during the initial movement of the pedal lever 62, this spring keeps the pins 5'! on the lever 68 against the left hand. ends of the slots 66.

Until the brake surfaces contact, therefore, the lever 68 swings with the pedal lever 62, the motionpbeing transmitted through the spring 69. -During this motion the an assembly including a pair of toggle struts 70,

two pairs of parallel levers ll, an abutment member 12, a servo spring 73, a pair of bell crank levers it, a pair of bell crank levers 15, and;

gripping elements referred to below. The outer ends of the toggle struts l6- bear respectively against pins it, each. of which extends between and is carried by one of the pair of levers H. The servo spring 13 tends" to swing the levers ll" '70 about the axes of the pins 16 but is prevented no connecting member 64 carries with it, as a unit, {I

prior to the engagement of the braking surfaces, produces horizontal components of force at the ends of the toggle struts I which cause the inner ends of the latter to tend to move to the right,

I but they are prevented from moving further to the. rightthan is shown in Figure 6 by theabutment member 12.

As soon as the braking surfaces engage each other there is resistance to further clockwise movement ofthe lever 68. The result is that continued movement of the pedal lever 62 causes flanges ,19 of the U-shaped gripping element I8 to bear hard against the pins 61 on the lever 68. Consequently, those parts whichhave hithereto moved as a unit with the connecting member 64, in the same relative positions, are caused to undergo relative movement. The bell crank levers I4 and I5 being pivoted to the gripping element I8, can no longer move to the left but remain stationary, bearing hard against pins 80 on ears 8| integral with the gripping element 18, while the connecting member 64 carries the pins 65 up to and beyond their dead-centre posiprimarily due to the servo spring I3 exerting a tions relatively to the pins I5, the servo spring I3 being slightly strained as the dead-centre position is approached. As soon as the dead-centre position has been passed, the horizontal components of the reactions at the outer ends of the struts ID are reversed so that the bell crank levers I4 and. 15 are swung about the trunnions 11 away from the pins 80. When this happens, a gripping shoe 82 lying within the U-shaped gripping element I8 is forced downwards so that a fixed, square rod 83 is gripped tightly between the shoe 82 and the clutch element I8. 'This swinging movement ofthe bell crank levers I4 and 15 about the'trunnions TI is transmitted to the gripping shoe 82 by pairs of links 84 and 85.

The links 84 connect pins 86, on the bell crank .levers I5, with pins 81 on the gripping shoe 82,

while the links 85 connect pins 88, on the bell crank levers I4, with pins 89 on the gripping shoe 82. p v

' As the inner ends of'the toggle struts 10 are moved to the left, so as to bring'them beyond their dead-centre positions, the slots '66 in the connecting member 64 traverse the pins 61 on the lever 68 until the right hand ends of the slots 66 bear against pins 61. The effort exerted on the pedal lever 62 is then transmitted directly through the connecting member 64 to the lever 68, the spring '60 remaining in its compressed state while the pressure between the braking surfaces is increased. The effort necessary to bring about this increase in pressure is, of course,

longitudinal thrust to theleft on the connecting member 64 through the medium of the toggle struts 10. The reaction of the servo spring I3,

which would otherwise simply tend to move the pairs of levers II to the right without transmitting any thrust through the connecting member 54, is taken by the gripping elements which are held firmly in engagement by the servo spring 13 gripping'mechanism in each of theconstructions of Figures 1 to 5 are causedby the drivers effort to engage before the servo springis moved to the dead-centre position, the toggle struts I0 in Figure 6 are moved to their dead-centre positions priorto the engagement of the gripping parts by the effort due tothe servo spring. Further, in each of the constructions of Figures 1 to 5, the

servo spring transmits its effort through the gripping mechanism while in the construction of Figures 6 to 8, the gripping mechanism simply takes the reaction of the servo spring. In addition, while in each of the constructions of Figures l to 5, the drivers effort is transmitted through the gripping mechanism after the parts of the. gripping mechanism have been caused to engage, in theconstruction of Figures 6 to 8 the effort exerted' on' the pedal lever 02 is transmitted directly tothe lever 68 through the connecting member 64 when the toggle struts have passed beyond their dead-centre positions.

The underlying principles of the mechanism shown in Figurev 9 are very similar to those of the mechanism of Figures 6 to 8." In Figure 9, a pedal lever 90 forms part of a toggle, the other link of which is a strut M which bears atone (end on a pin 92, carried by the, pedal lever ii0yand" at the other end one, pin- 03, carried by a lever 94. A servo spring95, before the braking surfaces are brought into contact, holds the toggle inthe position shown in Figure 9 with the proximal ends of the two toggle members 00 and 0! tending to move to the left but being prevented from doing so by the engagement, of a pin 05, on the pedal lever 90; with one end of a, slot 01' in a lever 98. The lever 94 is pivoted at its right hand end to a. bell crank lever 00 which, in turn, is

pivoted at 00 to aplate IIlI The latter is carried by links I02 and I03, pivoted at their upper ends to the casing containing the mechanism. The links I02 and I03 form with the, plate I 0| a parallelogram, linkage. The servospring 05 and the toggle members90 and BI are so arranged that during the initialmovement of the brake lever 90, prior to the engagement of thelbraking' surfaces, the servo spring and toggle simply act as if they were third and fourth links like the links I02 andl03. The initial movement of the brakelever 9B, therefore, simply results in the plate IOI swinging to the right on its links I02 and I03, whilethepedal lever 93, acting through a spring-loaded telesco-pic strut I0 3, swings a force-transmitting lever I05 in an anti-clockwise direction, the lever I05 being connected to the:

brake shoes through an appropriate linkagellike the linkage 203*2 I0'shown in Figures l3 to l5). As soon. as the braking surfaces engage, the

lever I05 can no longer swing in an anti-clockwise" direction, so that the continued-movement of the pedal lever 90 results in. thecompression of the;

spring I04, while the linkage, comprising the links I02 and I03 and the plateIElI areprevented from following the pedal lever 00 by the engagement of the'right hand end of theleverIm-with a pin I06 on the lever I05. The toggle is consequently straightened and the pin tzeventually passes over the dead-centre position relatively to {the pin 03 and the axis eta, of the pedal lever 90. I

' Before the toggle reaches the dead-centre position, there is a component'of force due to the servo spring 95 acting on the pivotal connection between the plate 04 and bell crank lever 99 in a direction such as to hold the latter against'a stop IIl'I on the plate IOI. As soon, however, as the toggle passes over the dead-centre position.

the direction of this component of force is reversed, so that the bell crank lever 99 is swung aboutthe axis IQII, and acting through a strut I08, causes afriction gripping elementIIiQ to press a rod IIII tightly against a second friction gripping element III. The rod III), which is pivoted at its right hand end to the casing, is thus tightly gripped between the gripping elements IE9 and III. 1

As soon as the pedal'lever 90 passes over the dead centre position, a pin IIZ thereon bears against a flat surface H3 on a strut member I I4 held in a strap I I 5 which embraces both the pin IE2 and a pine Ht, which is carried by the lever I05 and which serves as a pivotal bearing for the right hand end of the strut member I It. The effort supplied by the driver is, therefore, transmitted through the pin I I2 and strut member IM to the lever I135. As the engagingbraking surfaces are forced tightly together, the relaxing servo spring 95 surrenders its energy through the pin H2 and strut member IM, theeffort being transmitted through the strut 9 I. It will be appreciated that this effort which, of course, is applied when the pin 92 is to the right of its dead-centre position, would be wasted in swinging the plate IIJI to the left, were it not for the gripping elements which take the horizontal component of the reactiondue to the servo spring 95 at the pin 93.

When the drivers effort is removed, the strained brake linkage is, as in the previous examples, able to overcome the spring 95, so that the parts are returned to the positions shown in Figure 9, first by the relaxing linkage and then by the relaxing spring IM. spring IIM requires, perhaps, a little more explanation, especially as it will probably have been noticed that the lever 98, hearing on the pin I06, would be sufiicient to effect the initial movement, prior to the engagement of the braking surfaces, of the force-transmitting lever I05.

When the latter is held up as a result of the engagement of the braking surfaces, the reason why force can be exerted on the pedal lever ca, during continued movement of the latter prior to the passing of the dead-centre position, is due to the resistance of the spring IM and of the servo spring 95, the force being transmitted through these springs to hold the braking surfaces in contact. Now, as the servo-spring approaches its dead-centre position, i. e., its position of maximum stretch, its eifect decreases, but the effect of the spring HM increases as it is compressed. In other words, the spring IIM provides a rising-force resistance while the spring 95 provides a falling-force effort as itapproaches its dead-centre position. Thus, if the servo spring 7 I94 is, therefore, provided to remove this risk.

The rising-force resistance provided by the spring I64, combined with the falling-force characteristic of the servo spring 95, maintains a more or less constant or slightly rising-force to resist the relative displacement between the pedal lever QII and the force-transmitting lever I05. The spring The purpose of this 69 in the construction of Figures 6 to 8. functions similarly tothe spring I04 in Figure 9.

- In the mechanism of Figure 10 the brake pedal,

not shown, is fixed to a spindle II'I to which is fixed a pedal lever I I8. A pair of long levers H9 are mounted to swing freely about the spindle these two long levers.

mechanism are in the positions shown in Figure 10, and when the initialmovement of the pedal lever I I3 occurs for bringing the braking surfaces into contact, the pedal lever H8 is swung in an anti-clockwise direction, The pedal lever IIB is connected by a telescopic strut IZt'to a lever I2I 1 II9 bypivotally mounted on the long levers means of a journal Its. A servo spring I22 tends always to expand the telescopic strut I20,

and in the initial positionsv of the parts, tends to cause the lever I 2i to swing in a clockwise direction relatively to the long levers H9. The lever IZI is, however, prevented from swinging Y in this direction because a pin I23 thereon is in engagement with one of the long levers I I9. The

reaction exerted at the right hand end of the spring I22 also acts to hold the pin I23in en;

gagernent with. the lever H9 becausethe spring reacts against the pedal lever III; and thence,

through an extension I33 acts on the head I34 ofv a slidable rod I3I, thread ed at itsleit hand end. in a journal I35 so that this'pull also tends'to turn the lever iii, clockwise. Therefore, the pressure of the spring I22 at this phase of the operation keeps the clutch lever I25 as a rigid part of the long lever H9; The arrangement,

therefore, is such that during the initial anticlockwise movement of the pedal lever II 8, the

I long levers H9, the telescopic lever I29, and the lever 'IZI move as a unit with the pedal lever H3. The motion is transmitted to theb rake' shoes through. a force-transmitting lever I24 which ,is moved by the long levers H9 bearing upon rollers'I25 carried by a strut I28 formed with a fiat surface I27 that bears on a pin I28 The fiat surface I27 carried by the lever I24. forms part of the boundary of an aperture in which the pin I28 lies. is formed at the other end of the strut I26 and this contains a pin I29 carried by the pedal lever When the braking surfaces make contact, so

A similar aperture I30 that'the lever I24 experiences substantial resistance to further movement, the'pair of-long levers I I9 are held back by the rollers I25, downward movement of the. latter being prevented by the pin I25 bearing against the flat surface I 21.

Thus, continued movementof the brake lever IIII-takes place relativelyto the long levers II 9 and a spring I59 (equivalent to the spring I54 in Figure 9) is compressed while the right hand end" of the telescopic strut IN is moved to and beyond It may be mentioned" its dead-centre position. that the two telescoping parts of the strut I 20 are formed respectively with plane surfaces, I36 and I 3'! which respectively bear against a pin I38, on the pedal lever H8, and a pin I39, on the lever IZI. The dead-centre position, of course,

of the spindle II'I.

The pivotal connection between the lever I2I and the long arms H9 is provided by the journal Mt which passes through" slots I II in the long arms H9 and which carries a roller I42. The latter, under the horizontal component of the and which is held in tension so long as the servo spring I22 holds the pin I23 in the position shown in Figure 10, is released as soon as the strut I29 caused to carry the pin I38 over the dead-centre position, the line of action of the force exerted by the servo spring I29 passes above the axis of the ,journal I49 and beneath the axis of the spindle 1 that the servo spring I22 tends to move the pin I23 away from the lever II9. This anti-clock I IT. The turning moment due to the servo spring I22 upon the lever I2I, is then anti-clockwise, so

wise movement of the lever 'IZI actually takes place quickly, becaus'e'a spring I4 5, which is connected between the'levers III! and the lever IZI passes over the dead-centre position. The spring .I44 is very much weaker than the spring I22 although the line of action of the latter is con siderably closer to the axis of the journal I46, so that, in effect, the spring I44 only becomes effective when the moment due to the servo spring ;I22 is reversed. A gripping shoe 545, pivoted upon the clutch lever ,I2I is then brought sharply into engagement with the rail I43 under the combined influence of the two springs. anti-clockwise movement of the lever This I45 to wrap upon the rail I43 sothat the latter is firmly'gripped. If the rail I43. is mounted on rubber bushes, the slots I4I can be dispensed :with andthe journal I carried in plain journal bearings in the long arms H9. In this case the roller I42 and the gripping shoe I'will wrap'upon and grip the rail I43 equally well.

When the dead-centre position has been passed, the pin I29 on the pedal lever IIB bears against the lower fiat face of the aperture I35,

.,so that upon further movement of the pedal lever II8, the effort exerted by the relaxing servo spring I22 has a downwardly-directed vertical component which assiststhe driver injturning the lever I24 further in. an anti-clockwise direction, the force due to the driver and to the servo spring being transmitted through the strut I26. The reaction at the left hand end of the servo spring I22, which would otherwise simply 'cause this spring to swing the levers HQ n I clockwise direction instead of exerting a thrust through the strut I26, is taken by the gripping elements,

It will be noted that the pins I528 and IE9 and their associated plane surfaces I36 and I3! at the ends of thetelescopic strut I29 provide a. form of pivotal joint which is used extensively in the struts, and these have apertures which en-- gage the cylinders with suflicient looseness to allow the required degree of rolling to take. place;

In the case of the telescopic strut in Figure 10, these ears bear the reference numerals I I! and I48.

The reason why the rollersI25 are .mounted on one side of the strut I26 is to ensure that this strut shall take up the position shown in Figure I2 I, in effect, causes the roller I42 and the gripping shoe T removed from the brake pedal.

10 relatively to the other parts in the initial state of the mechanism. The object of thisis twofold. Firstly, it allows the clearances of the bearing surfaces between the strut I26 and the pins I28 and I29 to be in the nec'essary relative positions. When the pin I29 contacts with the.

flat 'orplane part of .the boundary of the aperture I39 the pins I28 and I29 roll on their respective plane surfaces under the force from the lever I I8 as the latter is movedto force the braking surfaces together. the strut I26 slightly to rotate in a clockwise direction. If the aforementioned clearance were not on the right sideto allow this clockwise rotation the pins I23 and I29 would be compelled to skid on their respective plane surfaces. Secondly, the placing of the rollers I25 causesthe pin I29 to enter a small recess I46 at one side of the aperture I39; when the parts are in the positions shown in Figure 10. Thepin I29 and recess I46 together provide a form ofcatch preventing too ready movementof the pedal lever I I8 from its initial position.

Figure 11 shows a modifiedform .of gripping arrangement for the mechanism of Figure 10.

In Figure 11 the rail I43 is replaced by two arcu ate strips I49 interleaved at their ends between resilient rubber blocks I591). A pad ofv friction material I5I is mounted to slide between the strips I49, and the long levers II9 carry pins I52 projecting between the strips I49 at each end of the pad I5I,so that the latter swings with the long levers H9. When the-lever IZI is swung in an anti-clockwise direction, the arcuate strips I49 are gripped tightly between the roller I42 and the'shoe I45, while the pad I5I is gripped tightly between the strips I49.

In the various constructions described above the inertia of the force-transmitting member and the linkage connecting the latter to the brake shoes might,.in the event of the driver giving the brake pedal a sudden jab, result in relative movementbetween thepedal lever and theforcetransmitting lever. before the braking surfaces contact, so that the clutch would be engaged prematurely. In order to eliminate this risk a weight may be provided, in such a manner as to ensure that the clutch shall not be operated and the servo spring caused to move over the deadcentre position until the braking surfaces contact, thereby eliminating any danger of the brake linkage being strained during the wrong part 01 the movement of the pedal lever, and the consequent risk of the parts not being returned to their initial positions when they drivers effort is For example, in the construction shown in Figure 2, the weight may be attached to extensions of the levers 2|,

these extensions being in a downward direction I In this case the inertia of beyond the spring 23. the weight would be such that, during the initial movement of the pedal lever in the application of the brakes, it would tend to keep the clutch shoes 25 separated in spite of the tendency of the inertia of the brake linkage to cause these shoes to grip the square tube 26. Again, in the. example shown in Figure 10, a counterweight may be mounted on an arm extending from'the telescopic strut to the right and in alinement therewith.

Figure 12 shows a modification of the construction shown in Figure 2. In Figure 12, a square tube I52a, equivalent to the tube 26 in This rolling action causes through the medium of a link I53, 2. bell crank initial movement, the bell crank lever I55, the

:ment with the square tube and the rollers I65 and continued clockwise movement of the pedal 1 the direction of the arrow, and prior to the engagement of the braking surfaces, a force-transmitting lever I5Ia, pivoted about a fixed axis I58, undergoes equivalent movement, the, pedal lever I5GA being connected to the lever I 5Ia lever I55, a shoe I51, a three-armed lever I55 and a link I54. The'bell crank lever I55 and the three-armed lever I55 are pivoted to the shoe I5? respectively at I63 and I54. During the three-armed lever I55, and the shoe I5! move as a unit to the left, without relative movement taking place between these parts, due to a spring I52 connected between the bell crank lever I55 and three-armed lever I56 holding stop projections I58 and I59 on these levers in contact with a stop surface I on the shoe I5'I. The link I53 and bell crank lever I55 are pivoted together at the axis of a roller I55 which is free to rotate independently of the link and the lever. Similarly, the link I55 and the three-armed lever I56 are pivoted together at the axis of a roller I which is free to rotate independently of the link and the lever. Thus, during the initial movement, prior to the contacting of the braking surfaces, the rollers I55 and I65 simply roll on the top of the square tube I52a, while the shoe I 57 is carried to the left and remains out of engage- I52a. When the braking surfaces contact, the force-transmitting lever I5I is held back as the pedal lever I50A continues to swing in a clockwise direction. The resultof this is that the rollers I65 and I66 separate slightly, while the bell crank lever I55 swings in an anti-clockwise direction, and the three armed lever I55 swings in a clockwise direction, the stop projections I58 and I59 moving upwards,-away from the stop surface I65. The

shoe I5! is therefore raised so that a layer of friction material on its supper surface engages the underside of the square tube I52a. The latter is thus gripped tightly between the shoe I51 I 66. Then, however,

lever I5tla causes the rollers I65 and IE6, the square tube I52a, and the shoe I57 to move to the left as a unit, causing the force-transmitting lever I5Ia. again to swing in a clockwise direction under the combined force exerted by the servo unit and by the effort of the driver.

As the shoe I5? is lifted into tight engagement with the square tube I52a, the spring IE2 is stretched. t will be seen that this spring is connected to the three-armed lever I55 at a point beneath the axis of the roller I 66, while it is connected to the bell crank lever E55 at the axis of the roller 55. The reason for this is to ensure that when the rollers I65 and 256 are caused to separate, the three-armed lever I55 swings in a clockwise direction before the bell crank lever 555 swings in an anti-clockwise direction. This is necessary because under the action of the forces to which they are subjected, the roller IE6 and the right hand end of the shoe Ililtend to wrap on to the square-tube I52a, while the roller I65 and left hand end of the shoe do not. If the bell crank lever I55 operated before the three-armed lever I55, there would be a risk of the square tube I52a being shot forward under theaction of the servo spring and releasing itself from-the gripof the roller E55 and left hand'end of the shoe I I5? during itsmov'ement; Obviously, in the arrangement illustrated, the three-armed lever I56 acts prior to the bell crank lever I55 because the momentexert'ed by the spring I52 about the axis 6 1 is less than that exerted by the spring about the three-armed lever E55, and the purpose of l the axis I63. c

It will be seen that a weight I5I is mounted on.

this weight is the sameas that of the weights. mentioned above that can be applied to the other illustrated constructions. In the event of the pedal lever Ilifla'beinggiven'a sharp initial move? surface I65, preventing the operation ofthe gripping elements and forcing the force-transmitting lever I5 id to follow the pedal lever I5ta until the braking surfaces contact.

the upper end of the ball crank lever I55 upwards beyond the roller I 65.

I claim:

.1 1 ment' such that the force transmitting lever I5I'd Q3; and the braking linkage (connecting the-forcetransmitting lever with the brake shoes) could 20 A'similar weight can," if necessary, be applied to an extension of the 1 bell crank lever I55, this extension extending from i. Mechanism for bringing two friction surfaces into engagement, increasing the pressure between' 1 V the surfaces when in engagement, and subse-;

quently separating the surfaces, said mechanism 3 comprising in cornbinatioman operating element,

a force-transmitting member adapted to be 019- eratively connected to one of the'friction surfaces and operatively connected to said operating element so as to be moved by a force applied to said operating element to bring the friction surfaces into engagement, a gripping mechanismconnect-- ed to be operated by movement of said operating element relatively to said force-transmittingmember when resistance to the movement of the latter is set up by engagement of the friction surfaces, and a servo spring arranged to be rendered effective, uponfurther movement of said operating element, to act throughsaid gripping mechanism and to expend energy on said force-trans- 5d mlttmg member in association with the force ape, 1

plied to said operating element toforce the surfaces tightly together. I I

2. Mechanism for bringing two friction'surfaces' into engagement, forcing the surfaces tightly to gether when in engagement to increase the fric-v tion therebetween, and subsequently separating the surfaces, said mechanism comprising in com-, bination a force-transmitting member adapted to be operatively connected to one of the friction surfaces, an operating element operatively con nected to said force-transmitting member to move v I said force-transmitting member to bring the friction surfaces together during a first movement; of said element and to undergo further movement relatively to said force-transmitting member while the latter is held substantially stationary-due to the reaction of the friction surfaces in engagement, a servo spring, and gripping mechanism,

operatively associated with said servo spring,

said force-transmitting member and said operat ing element, to render said servo spring operative,

upon the occurrence of the further movement ofsaid operating element, to assist the forcing.

together of the engaging friction surfaces during braking surfaces have contacted.

still furthermovement of said operating element. *3. Mechanism for bringing two frictionsurfaces into engagement, increasing the pressurebetween the surfaces. 'whenfin" engagement, and-subsequently separating. the surfaces,saidmechanism comprising in combination .anoperating element,

a force-transmitting memberadapte'dto beoperatively connectedtoyone ofv the friction surfaces and operatively connected to saidoperating element so as to be moved by-a force applied to. said operating element to bring the friction surfaces into engagement, a resilient connection between said operating element and said force-transmitting member, agripping mechanism having elements connectedto be brought into engagement witheach other bymovemen't of said operating element relatively to, said force transmitting member when resistance to movement of said forcetransmitting member is set up by engagement of the friction surfaces, and a servo spring opera:

tively. associated with said gripping mechanism an operating element connectedto act upon'said v force-transmitting member to bring (the braking surfaces into engagement, a gripping mechanism connected to be operated upon said force-transmitting member being held up bythe resistance setup due to the engagement of the braking surfaces while movement of said operating element continues, and a servo device, inclu ding a spring to provide the servo' force, normally free of said force-transmitting} member and in stable equilibrium but connected to be rendered capable of acting on said force-transmitting" member by means of said gripping mechanism so that said servo device can come into action only. when the 5. Mechanismfor bringing'two v faces into engagement, increasing. the pressure between thesurfaces when-,in engagement, and subsequently separating thesurface's, said m'ech anism comprising in combination anope'rating element, a force transmitting member adapted to beoperatively connected to'one of the'friction surfaces" and operatively connected to said (o der-'1 ating element so as to be movedby a force applied to said operating element to' 'bring the friction surfaces into engagement, a' gripping'mechan'ism' connected to be operated'by movement of said operating element relatively to saidforceetransmitting member when resistance to the movement of the latter is set up by engagement of the friction surfaces, and a servo mechanism including a lever device mountedfor movement from one side of a dead-centre positionto the other side thereof and also including a" servo spring inoperative engagement with said 1ever device and arrangedto hold said lever" device I slightly tooneside of thedead-centreposition while the frictionsurfaces arebrought into engagement, said servomechanism beingrendered, by operation of said gripping mechanism, eifective to expend energy, during the relaxing of said servo spring onthe other sideof thedead-centre position, on said force-transmiting member; and.

;1 8..Mechanism for bring two friction friction sur- .ment of said operating element.

*6. In mechanism :for controlling the engagement of braking surfaces, the combination of a force-transmitting member adapted to be operatively. connected to one of the braking surfaces, "an operating element connected to actupon said force-transmitting member to bring the braking surfaces into engagement, a gripping mechanism having elements connected to be brought into.

.engagementby the resistance set up uponthev engagement of the braking surfaces holding up said force-transmitting member while movement of said operating element continues, and a servo device, including a spring toprovide the servo force, normally free of said force-transmitting member and in stable equilibrium, but connected ,to said force-transmitting member by the engagement. of said gripping elements, said servo devicethusbecoming effective by acting, through said gripping elements, upon said force-transmitting member only when the braking has actually. begun. f7. In mechanism for controlling force-transmitting member adapted to be operthe engagementof braking surfaces, the combination of a s i '91 said lever device being caused to pass to said other side of the dead-centre position by further move,-

atively connected to one of the braking'surfaces,

an operating element connectedto act upon said force-transmitting member to bring the braking surfaces into engagement, a gripping mechanism havingselements connected to be brought into engagement by the resistance set up upon the engagement of the braking surfaces holding 'up said force-transmittingmember while movement of 'saidoperating element continues, and a servo device, including a spring to provide the servo force, connected to .be rendered capable of act-1 ing on said force-transmitting member only when said grippinglelements are broughtlinto engagement and provide an abutment against whichsaid servo. device. acts.

surfaces" into engagement, increasing the pressure between the surfaces when in engagement, and subsequently separating the surfaces, said mechanism .comprising in combination, an operating elem,ent, a force-transmitting member adapted to,

be operatively; connected to one of the friction surfaces and operatively connected to said operating element so as, to be moved by a force applied tosaid operating element to bring the friction surfaces,.into engagement, a gripping.

mechanism, connected to be operated by movement of said operating element relatively to said forceetransmitting member when resistance to the movement of the latter is set up by engageanism including a lever device mounted for move.- ment from one sideto the other ofa deadcentre position and also including a" spring in ment of the friction surfaces, and a servo'meche operative engagement with said lever device and arranged to hold said lever device slightly to one-sidelof the dead-centre position while the surfaces are brought into engagement, sai dservo" mechanism being so operatively associated with said g'rippingmechanism and the latter being. so operatively associated with said force-transmitting member'that, subsequently to the engage ment of said gripping mechanism said lever de Vice can be caused to pass over' the dead-centre position whereupon said spring can, in relaxing,

augment the" force applied to said operating element to increase the pressure between the frictionisurfaces'. I

' mitting member adapted to be operatively connected to one of the friction surfaces, a resilient connection between said operating element and said force-transmitting member whereby said 9. In mechanism for bringing two friction surfaces into engagement, increasing the pressure between the surfaces when in engagement, and subsequently separating the surfaces, the combination of an operating element, a force-transoperating member may be caused to move said [force-transmitting member to bring the friction surfaces into engagement and may then be moved further While said force-transmitting member is held substantially stationary by the resistance set u bythe engaging friction surfaces, a lever device mounted for movement from one side to theother ofa dead-centre position, a servo spring operatively connected to said lever device and arranged to .hold said leverslightly to one side of the, dead-centre position while the friction surfaces are brought into engagement, and a grippring mechanism. operatively associated with said operating element, said force-transmitting. member and said lever device, said gripping mechanism being engaged to provide a connection between said lever device and said force-transmit,-

ting member by said further movement of said operating'element and arranged, upon still further movement of said operating element, to carry said lever device over the dead-centre position, thereby rendering said servo spring capable of expending energy on said force-transmitting member.

10. In mechanism for bringing two friction surfaces into engagement, increasing the.pressure between the surfaces when in engagement, and subsequently separating the surfaces, the

combination of an operating element, a forcetransmitting member adapted to be operatively connected to. onexof. theifriction surfaces, aresilient connection between said operating element and said force-transmitting member whereby said operating member may be caused tomove said force-transmitting member to bring the friction surfaces into engagement. and may then be moved further while said'force-transmitting member is held substantially stationary by the resistance setup by the engaging friction sur-' faces, a lever device mounted for movement from one side to the other of [a dead-centre position, a servo spring operatively connected to said lever device and arranged to hold said lever device slightlyv to one side of the dead-centre position while the friction surfacesare brought'into engagernent, said lever device being soassociated with said operating element and with said forcetransmitting member as to be forced over the dead-centre position by said further movement of said operating element, and a gripping mech- .anism connected to be operated by such forcing "'of said operating element overthe dead-centre position and adapted to provide, when so operated, an abutment against which said servo spring canreact in relaxing to expend energy in assistI- ing the further movement of said force-transmitting member to increase the pressure between.

the engaging friction surfaces.

11. Mechanism for controlling the engagement of friction surfaces, said mechanism comprising in combination, a servo unit, a pivotally-mounted operating element,. a pivotally-mounted forcetransmitting member, a linkage including a resilient element connecting said operating element with. said forcertra'nsmitting member and arranged to move as a rigid unit so long" as said,

force-transmitting member is free tobe moved .by said operating element but to undergo .de-

formation .whensaid operating element is moved while movement of said force-transmitting mem ber is resisted, a.gripping mechanism comprising 1 'a'gripping element connected to' said servo. unit and gripping shoes mounted ons'aid linkage and arranged .togrip said gripping element when'said deformation occurs was to provide with said linkage and said clutch element an assembly. whereby further motion of said operating element is transmitted to said force-transmitting member and to said servo unit, the latter being arranged thereupon to assist the further movement'of said force-transmitting member during still further motion of said operating element.

12..The invention of claim 11 in combination with a. further resilient element, connected be-- tween said gripping element and said servo unit,

permitting movement of said gripping element in the event of said servo unit being disabled.

13. Mechanism forcontrolling the engagement of friction surfaces, said mechanism comprising in combination a force-transmitting member pivotally mounted about an axis, an operating element pivotally mounted about the same axis,

means interposedbetween said element and said. memberoperative to cause relative movement along said axis between said element and said member when relative movement about said axis occursbetween the same, a further element, a

gripping element pivotally mounted about said.

axis and interposed between said operating ele ment and said furtherelement to be gripped therebetween upon the occurrence o'fsaid rel'a tive movement along said axis, a lever pivotally connected to said gripping element'and mounted' I- v for movement from one-side to the' other 'ofa dead-centre position, and a servo spring operatively connected to said lever and adapted, when the latter ison one side of the dead-centreposi 14. -Mechanism*for controlling the engagement] tion, to transmit movement through said gripping connection toone of the SUI bringing the friction surfaces into engagement, a

strut mounted to provide a rigid connection be-* tween said operating element and said force-' transmitting member after saidspring has been" strained due to the resistance set up by the engaging friction surfaces impeding movement; of .said force-transmitting member during contin'-' ued movement of said operating element,'a gripping mechanism,alever device connected between said clutch device. and said operating element,

said lever device being mounted for movement-- I from one side to the other of a dead-centre position, and a servo spring mounted to holdsaid leverdevice onione side of the dead-centre posi-I; 5 'tion, said lever device being so operatively asso-E" i, when=the latter is impeded, said lever device .is

ciated withsaid force-transmitting member that forced over the dead-centre position by the continued. movement of. said operating member whereby said grippingmechanism is engaged to" I5 provida an abutment against 4 which said servoa spring acts in relaxing to expend "energy on said operating element during still further movement between said operating memberf and said force--' transmitting member :to transmitmovement of the former to the latter'forjbringing the friction surfaces into engagement; a strut mounted to provide a, rigid connection between said operating element and said force-transmitting member after said spring has been strained due to the resistance set up by the engaging irictijon'surfaces impeding movement of said fo'rce-transmitting membenduring; continued movement of said operatingelement, 'a gripping mechanism, a leven-device connected between said gripping,

devicebeing mounted for movement from one mechanism and said operating elementsaid lever v side to the other of [a dead-centre position, a-

servo spring-mounted to; hold said lever device on one side ofthe dead-centre position, said lever-v;

device being so" operatively associated with said force-transmitting member that when the latter of said operating member, whereby said gripping mechanism is vengaged .to provide an abutment ,is'fimpeded, said lever device is forced over the dead-centreposition'by the oontimied movement against which said servo spring acts inrelaxing to expend energy'on said operating velementduring still further movement ofthe latter, and a subsidiary spring connected to oppose said servo spring until said lever device'is forced over the dead-centre position and thereafter toassistfinj the engagementof said-gripping mechanism.

GEORGE CARWARDINEY -0 i 

